Defining true propagation patterns of underwater noise produced by stationary vessels

The study of underwater vessel noise over the past sixty years has predominantly focused upon the increase in ambient noise caused by the propulsion mechanisms of large commercial vessels. Studies have identified that the continuous rise of ambient noise levels in open waters is linked to the increase in size and strength of anthropogenic sound sources. Few studies have investigated the noise contribution of smaller vessels or ambient noise levels present in coastal and in-shore waters. This study aimed to identify the level of noise common to non-commercial harbors by studying the noise emissions of a diesel generator on board a 70m long sailing vessel. Propagation patterns revealed an unconventional shape (specific to the precise location of the noise source on board the vessel), unlike those of standard geometric spreading models, as typically assumed when predicting vessel noise emission. Harbor attributes (including water depth, ground sediment and structural material components) caused for altered level and frequency characteristics of the recorded underwater noise, and were correlated to the sound measurements made. The measurements (taken in eight harbors around Northern Europe) were statistically analyzed to identify the primary factors influencing near-field sound propagation around a stationary vessel.